TIDALLY INFLUENCED VERTICAL MIGRATION AND IN SITU OBSERVATIONS OF THE SCYPHOMEDUSA CHRYSAORA QUINQUECIRRHA

Vertical migration of zooplankton populations is a common phenomenon and may be important for predator avoidance, spawning and vertical nutrient flux in oceanic systems. There are numerous examples of diel vertical migration of gelatinous zooplankton, but reports of tidally influenced migrations are rare. I recorded the volumetric density of the scyphomedusa Chrysaora quinquecirrha in surface waters of two tributaries of Chesapeake Bay every 12 hours throughout a field season. Data was analyzed using time series and spectral analysis techniques. Increased surface abundance of medusae was circatidal with a period equal to the O1, principal daily lunar cycle. Maximum abundance coincided with the new and full moons. A precipitous decline in abundance of medusae at one of the sites in July coincided with the experimentally determined upper lethal temperature for this species. However, the reappearance of medusae after a decline in the ambient temperature was not accompanied by a change in the size structure of the population suggesting that the disappearance was not due to mortality but rather to medusae seeking refuge in deeper, cooler waters.